Fluororesins have excellent characteristics as heat resistance, corrosion resistance, nonadhesiveness and lubricity, so that those are used in various fields of application. Concerning how to use a fluororesin, mention may be made, for example, of the method which comprises applying a coating containing the same to substrates to which the same is desired to be applied, for example rolls for OA equipments, belts for OA equipments and the like, drying, if necessary, the applied coats obtained, and baking the same to form fluororesin-containing coating films.
For example, ethylene-tetrafluoroethylene copolymers (ETFEs), which are excellent in such characteristics as corrosion resistance and heat resistance, are used in molding materials and coatings, among others, and, as coatings, they are used mostly in the form of powder coatings. Generally, such ETFE-based coatings do not require the use of a primer as an undercoat to be applied in advance to materials to be coated and, thus, they can be used in a simple and easy manner, hence are in general use.
On the other side of such expediency, ETFE-based coatings have a problem, namely they are poor in adhesion to materials to be coated. For example, when an ETFE-based coating is applied to form a relatively thick coating film, the coating films readily undergo cracking; therefore, the increase in coating film thickness is restricted. Further, even when an ETFE-based coating has a certain extent of adhesion after application, they still have a problem. Thus, when placed under severe conditions, for example in hot water or at elevated temperatures, the adhesion decreases, with the result that the coating films may undergo cracking or may peel off from the materials to be coated; the stable adhesion may thus be missed to vary widely according to the environment in which the coated articles are placed.
While ETFE-based coatings are also applied to materials to be coated, which are made of a chromium-containing metal, such as stainless steel in certain instances, the application of those coatings to such articles poses a problem since such chromium-containing metal promotes the decomposition of ETFEs.
To solve those problems, the advent of a primer for ETFE-based coatings has become desirable. Known as a primer for ETFE-based coatings is a PPS-based primer which comprises polyphenylene sulfide (PPS) and a polyamideimide resin. However, there is no solvent available for dissolving PPS and, therefore, the PPS-based primer is used in the form of a solid, which is difficult to spread onto materials to be coated evenly and uniformly; as a result, air gaps are readily formed between the primer and substrate and the above-discussed problems about adhesion and processability cannot be fully solved.
In the art, coating films containing a fluororesin other than ETFEs are formed, according to the properties of the substrate used as a material to be coated, by the two-coat technique comprising coating the substrate with a primer containing a binder resin in advance, for the purpose of increasing the adhesion to the substrate, and then applying the fluororesin-containing coating.
This two-coat technique requires the steps for primer coat formation, namely applying the primer to the substrate, drying and heating, for instance, prior to application of the fluororesin-containing coating. Therefore, the advent of a coating process capable of simplifying the process and saving the energy has also been desired.
Available as a method for process simplification and energy saving is the one-coat technique comprising one single application of a coating containing both the fluororesin and binder resin. The intention of this technique is to form a coating film comprising a primer coat and a fluororesin, which have so far been formed by the two-coat technique, by applying one coating.
The coating film obtained by the one-coat technique comprises the binder resin disposed mainly on the substrate side and the fluororesin disposed mainly on the coating film surface side remote from the substrate, so that both the resins have respective concentration gradients from the substrate side to the surface side of the coating film. A coating capable of giving coating films having such concentration gradients is sometimes referred to as “gradient function type coating” as well.
Fluororesin-containing, gradient function type coating can thus form coating films with the fluororesin and binder resin each being disposed in a gradient manner and, therefore, they have been used in a wide range of application fields to make use of the nonadhesiveness, lubricity and other characteristics of the fluororesin.
However, the conventional one-coat technique has a problem; the fluororesin-containing coating film surface layer with a satisfactory thickness cannot be formed thickly enough, so that not only the corrosion resistance, nonadhesiveness and other excellent characteristics intrinsic in the fluororesin cannot be exhibited to a full extent but also poor durability results.
When a conventional gradient type coating for the one-coat technique is used, a layer comparable in thickness to the fluororesin-containing coating films obtainable by the two-coat technique is obtained by a plurality of times of recoating with the gradient function type coating for the one-coat technique, or in case of coating tubes or the like, but such a layer cannot be obtained by one single application of the conventional gradient type coating for the one-coat technique.
Furthermore, in the state of the art, it is impossible, in the coating films obtained, to dispose the fluororesin mainly on the substrate side and the binder resin on the coating film surface side remote from the substrate. Thus, the advent of a coating by which such disposition is possible has been desired for expanding the application range.